A conventional combustible gas turbine engine includes a compressor, a combustor, and a turbine. The compressor compresses ambient air. The combustor combines the compressed air with a fuel and ignites the mixture creating combustion products defining a working gas. The working gases travel to the turbine. Within the compressor are a series of rows of stationary vanes and rotating blades. Each pair of rows of vanes and blades is called a stage. The rotating blades are coupled to a shaft and rotor disc assembly.
For each row of blades, a separate rotor disc is provided. The rotor discs form part of the shaft and rotor disc assembly. One or more of the rotor discs is provided with a dovetail slot extending 360 degrees about the disc so as to receive dovetail bases or roots of blades. For each row of vanes, a retention casing fixedly coupled to a housing of the gas turbine engine is provided. One or more of the retention casings is provided with a dovetail slot so as to receive dovetail bases or roots of vanes.
In order to frictionally hold a root of a blade in the dovetail slot, a solid shim is provided within a cavity defined by recesses in the blade root and the rotor disc. Similarly, in order to frictionally maintain a root of a vane in a dovetail slot, a shim is provided within a cavity defined by recesses in the vane root and the retention casing. If, for example, a range of cavity sizes varies, due to tolerances, from 15.00 mm to 15.30 mm and a minimum allowable gap within the cavity is 0.05 mm, a plurality of solid shims would need to be available during assembly of the blades with the rotor discs and the vanes with the retention casings, with each solid shim corresponding to a particular cavity size. For example, six shim height sizes (15.00 mm; 15.05 mm; 15.10 mm; 15.15 mm; 15.20 mm; and 15.25 mm) would be needed for the cavity size tolerance range of 15.00 mm to 15.30 mm. Hence, during assembly of a blade in its dovetail slot, a solid shim of an appropriate size would be selected from the six available sizes and inserted into the recess. Likewise, during assembly of a vane in its dovetail slot, a solid shim of an appropriate size selected from the six available sizes would be selected and inserted into the recess.
Instead of a solid shim, a thin wave-shaped spring shim may be provided. Such a shim is disadvantageous because it is susceptible to loosing its spring force, allowing relative motion of a corresponding blade or vain resulting in wear.